Scroll pumps are disclosed in U.S. Pat. No. 801,182 issued in 1905 to Creux. In a scroll pump, a movable spiral blade orbits with respect to a fixed spiral blade within a housing. The configuration of the scroll blades and their relative motion traps one or more volumes or "pockets" of a fluid between the blades and moves the fluid through the pump. The Creux patent describes using the energy of steam to drive the blades to produce rotary power output. Most applications, however, apply rotary power to pump a fluid through the device. Oil-lubricated scroll pumps are widely used as refrigerant compressors. Other applications include expanders, which operate in reverse from a compressor, and vacuum pumps. To date, scroll pumps have not been widely adopted for use as vacuum pumps, mainly because the cost of manufacture for a scroll pump is significantly higher than for a comparably sized oil lubricated vane pump.
Scroll pumps must satisfy a number of often conflicting design objectives. The scroll blades must be configured to interact with each other so that their relative motion defines the pockets that transport, and often compress, the fluid within the pockets. The blades must therefore move relative to each other, with seals formed between adjacent turns. In vacuum pumping, the vacuum level achievable by the pump is often limited by the tendency of high pressure gas at the outlet to flow backwards toward the lower pressure inlet and to leak through the sliding seals to the inlet. The effectiveness and durability of the scroll blade seals are important determinants of performance and reliability.
Sealing means for scroll-type apparatus, including a seal element backed by an elastomeric member, are disclosed in U.S. Pat. No. 3,994,636 issued Nov. 30, 1976 to McCullough et al. A seal configuration including a sealing strip biased by a silicone rubber tube is disclosed in U.S. Pat. No. 4,883,413 issued Nov. 28, 1989 to Perevuznik et al. A seal arrangement for a scroll-type vacuum pump, including a seal element and an elastomer seal loading bladder which may be pressurized, is disclosed in U.S. Pat. No. 5,366,358 issued Nov. 22, 1994 to Grenci et al. A scroll-type pump having a seal configuration, including a seal member and a backup member of a soft porous material, is disclosed in U.S. Pat. No. 5,258,046 issued Nov. 2, 1993 to Haga et al. Additional seal configurations for scroll-type apparatus are disclosed in U.S. Pat. No. 4,730,375 issued Mar. 15, 1988 to Nakamura et al. Prior art tip seals typically include a seal element that forms a sliding seal and an energizer element that forces the seal element against an opposing surface.
Tip seals critically affect the performance and reliability of dry scroll pumps. The tip seal is typically mounted in a groove machined into the top edge of a scroll blade. The seal must effectively block gas leakage across the seal (transverse to the seal) as well as axially along the tip seal groove. Leakage in either direction allows gas to travel back toward the pump inlet. The seal must provide adequate sealing for long periods of time (typically more than 9000 hours) with little wear, minimal friction and over a range of operating temperatures and pressures. The tip seals in prior art scroll-type vacuum pumps have a number of disadvantages that relate to elastomeric material properties, economically achievable machining tolerances and conflicting requirements of low leakage across and down the seal. Common elastomers such as rubber, Buna N and Viton are incompressible materials, i.e., the material density remains essentially constant under compressive stresses. Squeezing a cube of these materials vertically results in the material bulging out horizontally. For an elastomer seal located in a groove and having no space in which to deform, the seal will support very high vertical forces with essentially no vertical deformation. Consequently, to completely fill a seal groove under the light pressures required for low friction and long life, the dimensions of the seal, the seal groove and the clearance to the opposing scroll blade must be very tightly controlled. As a practical matter, tradeoffs must be made with solid elastomers as to how well the seal groove can be blocked. This limits pump performance.
Solid elastomers such as Viton, Buna N and molded silicones are also too stiff to use as seal energizer elements in a practical scroll pump. A typical modulus of elasticity for these materials is 200 to 700 pounds per square inch (psi). To limit frictional heating within the pump, the contact pressure must be kept low, ideally less than about 5 psi. If the elastomeric portion of the seal is 0.1 inch thick, then a 5 psi loading is achieved with Buna N with a deflection of only 0.001 inch. Tolerances within the pump must be held extremely tight to consistently achieve a 5 psi loading. Seal loading would change substantially with seal wear and with thermal expansion of scroll components as the pump operates.
One commercially available dry scroll vacuum pump uses unsintered Teflon paste as a seal energizer element. A useful attribute of Teflon paste is that it is a non-homogeneous material. A fraction of the material is air and, therefore, its bulk density can be increased by compaction. When the seal is pressed into the tip seal groove, the elastomer simultaneously yields and compresses to fill the seal groove nearly completely. The material takes a permanent set but, when released, springs back very little. This effectively blocks transverse leakage under the seal as well as along the tip seal groove. The energizer compensates for dimensional variations by deforming and compressing more or less without great variation in force. This is in contrast to a solid elastomer, which greatly resists deformation when dimensionally confined.
The design using a Teflon paste energizer element, however, has several disadvantages. When the scroll pump is started, its internal components gradually heat up due to friction and work performed on the gas being pumped. The Teflon paste expands in the groove relative to the surrounding metal and forces the seal surface against its counterface. When a new seal is first run, the Teflon paste compresses a bit further, taking a new permanent set. The proper initial paste density, width and thickness are adjusted, so that adequate sealing force is available at normal operating temperatures. Consequently, elevated temperature is necessary to ensure sufficient force to properly energize the seal. The energizer element must be in a thermally expanded state to function properly. Scroll pumps using this type of paste elastomer and started at low ambient temperatures often exhibit poor base pressure for many minutes until the pump and seals have warmed up. This behavior is unacceptable for some applications such as, for example, portable leak detection systems.
Another disadvantage of the Teflon paste elastomer is a loss of seal energizing force due to wear. Over time, both the seal and the counterface wear and become thinner. The wear is small, on the order of 0.003 inch per year of operation. However, after about a year, the thermal expansion of the Teflon paste is no longer sufficient to force the seal against the counterface. A degradation of pump base pressure results from increased leakage across the top of the seal. Although a large amount of seal material remains, the seals must be replaced.
A final disadvantage of the Teflon paste is that it is quite expensive. The material required to make seals for one pump costs about forty dollars.
Accordingly, there is a need for improved tip seal configurations for scroll-type vacuum pumps.